Refactored modules and reexports

[phsym]

• Reexported types used from csv crate
• Moved CSV stuff in dedicated module
• Reexport cell::Cell and row::Row into crate's root
• cell and row modules are now private

[codecov]

Codecov Report

Merging #89 into master will decrease coverage by 0.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   84.42%   83.83%   -0.59%     
==========================================
  Files           5        5              
  Lines        1220     1157      -63     
==========================================
- Hits         1030      970      -60     
+ Misses        190      187       -3

Impacted Files	Coverage Δ
src/cell.rs	85.58% <ø> (ø)	⬆️
src/row.rs	90.96% <ø> (ø)	⬆️
src/lib.rs	85.07% <ø> (-1.26%)	⬇️

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